Metal–halide perovskite nanocrystals (NCs) have emerged as suitable light‐emitting materials for light‐emitting diodes (LEDs) and other practical applications. However, LEDs with perovskite NCs undergo environment‐induced and ion‐migration‐induced structural degradation during operation; therefore, novel NC design concepts, such as hermetic sealing of the perovskite NCs, are required. Thus far, viable synthetic conditions to form a robust and hermetic semiconducting shell on perovskite NCs have been rarely reported for LED applications because of the difficulties in the delicate engineering of encapsulation techniques. Herein, a highly bright and durable deep‐blue perovskite LED (PeLED) formed by hermetically sealing perovskite NCs with epitaxial ZnS shells is reported. This shell protects the perovskite NCs from the environment, facilitates charge injection/transport, and effectively suppresses interparticle ion migration during the LED operation, resulting in exceptional brightness (2916 cd m−2) at 451 nm and a high external quantum efficiency of 1.32%. Furthermore, even in the unencapsulated state, the LED shows a long operational lifetime (T50) of 1192 s (≈20 min) in the air. These results demonstrate that the epitaxial and hermetic encapsulation of perovskite NCs is a powerful strategy for fabricating high‐performance deep‐blue‐emitting PeLEDs.

Light-emitting diodes (LEDs) are the rapidly developing core components of current display and lighting technology. Metal halide perovskite nanocrystals (MHP NCs) have recently been used as the deep-blue-light-emitting component in LEDs and are considered to have the greatest potential for growth in practical applications. However, the vulnerability of MHP NCs to the environment and the ion migration during the operation of LEDs pose formidable obstacles to the practical application of MHP NCs. Herein, we show that mixed-halide CsPb(Br1-xClx)3 NCs enclosed by epitaxially grown ZnS shells (CPBC/ZnS) are integral to ensuring a stable perovskite-based deep-blue-light-emitting diode (PeLED). We found that epitaxial ZnS shells protect the MHP NCs from the environment, and that the interparticle ion migration between MHP NCs could be effectively suppressed during LED operation, affording an exceptional external quantum efficiency (EQE) of 3.63% at an emission peak of 451 nm and a maximum luminance of 1687 cd m-2. Our results demonstrate that the epitaxial encapsulation of MHP NCs is a powerful strategy for the fabrication of high-efficiency, high-stability PeLEDs with a deep-blue emission.